The inventors of this invention invented a device for intermittently feeding powder to a piping for conveying the powder. That device comprises a pressurized tank that is a pressure vessel, and which tank has a tank body that has a port for loading in an upper wall and a port for discharging in a lower wall, a first check valve that is provided beneath the port for loading and is vertically moved so as to be upwardly moved by means of compressed air to close the port for loading, and a second check valve that is provided beneath the port for discharging and is vertically moved so as to be upwardly moved by compressed air to close the port for discharging. The device also comprises a hopper that stores the powder to be conveyed and has a port for discharging the powder at the bottom, which port faces the port for loading. The device also comprises a chute that connects the port for discharging the powder of the hopper with the port for loading of the tank body. The device also comprises a valve casing, one end of which is hermetically attached to a lower outer surface of the tank body so as to enclose the port for discharging of the tank body, and the other end of which is connected to a proximal end of the piping for conveying the powder (see Japanese Patent No. 4893993).
However, by the device for feeding powder to the piping that is configured as above, a differential pressure between the pressurized tank and the piping for conveying the powder must be high, for example, 50 kPa or more, so as to activate the second check valve. This is because the second check valve has a weight to match a robust structure for closing the port for discharging and for supporting the powder on the port. Since the differential pressure must be high in the second check valve, the second check valve may often open before the compressed air has been completely exhausted from the pressurized tank, under some conditions (e.g., the timing of pressurization, or the amount of the powder to be conveyed). If so, air or conveyed material (powder) flows back from the piping for conveying the powder to the pressurized tank so that the compressed air is wasted, i.e., without being used for conveying the powder. That has been a problem.
When powder is conveyed from a plurality of the pressurized tanks to one piping for conveying the powder, air or conveyed material may flow from one pressurized tank to another pressurized tank. This has been a problem. To solve this problem, a measure may be proposed to simultaneously pressurize the plurality of the pressurized tanks. However, a problem would occur in that the plugs of the powder being conveyed may concentrate or in that fluctuations of momentum may become large so that a vibration or noise in the piping becomes large. Further, a fluctuation in the consumption of the compressed air may become large so that a measure to absorb it would cause the cost to increase. Further, if some of the pressurized tanks have the flow of air or material suspended, air or conveyed material may flow to the suspended pressurized tanks until the pressure in the piping for conveying the powder achieves a level sufficient to activate the second check valve. Thus a device or a procedure to separate the suspended pressurized tank from the piping for conveying the powder is required, resulting in an increase in the cost.